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    SubjectRe: [PATCH] fs: Fix busyloop in wb_writeback()
    On Thu, Sep 17, 2009 at 01:22:48AM +0800, Jan Kara wrote:
    > If all inodes are under writeback (e.g. in case when there's only one inode
    > with dirty pages), wb_writeback() with WB_SYNC_NONE work basically degrades
    > to busylooping until I_SYNC flags of the inode is cleared. Fix the problem by
    > waiting on I_SYNC flags of an inode on b_more_io list in case we failed to
    > write anything.

    Hi Jan,

    I reconfirmed that this patch fixed the busy loop in wb_writeback()
    path. I also noticed that balance_dirty_pages() looped a lot. Given
    that balance_dirty_pages() schedules in between, it cannot be called
    busy loop. But the loop interval is about 0.01s, which is shorter than
    the expected 0.1s. This is a 2-core CPU, so I guess the cp process got
    re-scheduled immediately, long before it timed out.

    [ 491.440702] requeue_io() +457: inode=39381
    [ 491.442360] mm/page-writeback.c +540 balance_dirty_pages(): comm=cp pid=3302 n=0
    [ 491.444257] global dirty=51770 writeback=18201 nfs=0 flags=_M towrite=1536 skipped=0
    [ 491.445856] requeue_io() +457: inode=39381
    [ 491.446543] mm/page-writeback.c +540 balance_dirty_pages(): comm=cp pid=3302 n=0
    [ 491.448346] global dirty=51770 writeback=18201 nfs=0 flags=_M towrite=1536 skipped=0
    [ 491.460535] requeue_io() +457: inode=39381
    [ 491.462206] mm/page-writeback.c +540 balance_dirty_pages(): comm=cp pid=3302 n=0
    [ 491.463574] global dirty=51894 writeback=18077 nfs=0 flags=_M towrite=1536 skipped=0
    [ 491.474072] requeue_io() +457: inode=39381
    [ 491.475416] mm/page-writeback.c +540 balance_dirty_pages(): comm=cp pid=3302 n=0
    [ 491.477220] global dirty=52049 writeback=17922 nfs=0 flags=_M towrite=1536 skipped=0
    [ 491.481859] requeue_io() +457: inode=39381
    [ 491.482618] mm/page-writeback.c +540 balance_dirty_pages(): comm=cp pid=3302 n=0
    [ 491.485026] global dirty=52049 writeback=17798 nfs=0 flags=_M towrite=1536 skipped=0
    [ 491.496843] requeue_io() +457: inode=39381
    [ 491.499001] mm/page-writeback.c +540 balance_dirty_pages(): comm=cp pid=3302 n=0
    [ 491.500658] global dirty=52173 writeback=17674 nfs=0 flags=_M towrite=1536 skipped=0
    [ 491.508540] requeue_io() +457: inode=39381
    [ 491.509490] mm/page-writeback.c +540 balance_dirty_pages(): comm=cp pid=3302 n=0
    [ 491.511300] global dirty=52297 writeback=17674 nfs=0 flags=_M towrite=1536 skipped=0
    [ 491.517728] requeue_io() +457: inode=39381
    [ 491.518752] mm/page-writeback.c +540 balance_dirty_pages(): comm=cp pid=3302 n=0
    [ 491.521498] global dirty=52421 writeback=17550 nfs=0 flags=_M towrite=1536 skipped=0
    [ 491.525881] requeue_io() +457: inode=39381
    [ 491.527339] mm/page-writeback.c +540 balance_dirty_pages(): comm=cp pid=3302 n=0
    [ 491.528730] global dirty=52421 writeback=17550 nfs=0 flags=_M towrite=1536 skipped=0

    Thanks,
    Fengguang


    > CC: Wu Fengguang <fengguang.wu@intel.com>
    > Signed-off-by: Jan Kara <jack@suse.cz>
    > ---
    > fs/fs-writeback.c | 19 ++++++++++++++++++-
    > 1 files changed, 18 insertions(+), 1 deletions(-)
    >
    > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
    > index 8e1e5e1..c59d673 100644
    > --- a/fs/fs-writeback.c
    > +++ b/fs/fs-writeback.c
    > @@ -706,6 +706,7 @@ static long wb_writeback(struct bdi_writeback *wb,
    > };
    > unsigned long oldest_jif;
    > long wrote = 0;
    > + struct inode *inode;
    >
    > if (wbc.for_kupdate) {
    > wbc.older_than_this = &oldest_jif;
    > @@ -747,8 +748,24 @@ static long wb_writeback(struct bdi_writeback *wb,
    > * If we ran out of stuff to write, bail unless more_io got set
    > */
    > if (wbc.nr_to_write > 0 || wbc.pages_skipped > 0) {
    > - if (wbc.more_io && !wbc.for_kupdate)
    > + if (wbc.more_io && !wbc.for_kupdate) {
    > + if (wbc.nr_to_write < MAX_WRITEBACK_PAGES)
    > + continue;
    > + /*
    > + * Nothing written. Wait for some inode to
    > + * become available for writeback. Otherwise
    > + * we'll just busyloop.
    > + */
    > + spin_lock(&inode_lock);
    > + if (!list_empty(&wb->b_more_io)) {
    > + inode = list_entry(
    > + wb->b_more_io.prev,
    > + struct inode, i_list);
    > + inode_wait_for_writeback(inode);
    > + }
    > + spin_unlock(&inode_lock);
    > continue;
    > + }
    > break;
    > }
    > }
    > --
    > 1.6.0.2


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